Fermentation is an art that began before recorded history. It was, however, only in 1866 that Pasteur published his works on wine fermentation, analyzed the reasons for spoilage, and prescribed an appropriate treatment for wine making.
Traditionally the fermentation takes place with the wild yeasts that occur on all fruits and berries. Fermentation with pure culture yeasts is used more often today in order to consistently produce good quality wines. All these general aspects of wine making and characterizations of the yeasts are presented in "Biotechnology" edited by H-J Rehm and G Reed Vol. Food and Feed Production with Micro-organisms, Verlag Chemie or in Prescott & Dunn's Industrial Microbiology 4th edition, chapter 9, 1982 edited by G. Reed, the disclosure of which in incorporated herein by reference.
Traditional batch fermentation is time consuming. A typical time for the alcohol fermentation in wine making normally requires at least 20-50 days. Although a continuous process with suspended yeast might be thought to speed this process it is difficult to operate and maintain free from microbial contamination. Moreover, to speed up the fermentation rate, the yeast cell concentration in the fermenting must should be increased while on the other hand the ethanol inhibition should be minimized. In a batch fermentation, for example with apple juice, the yeast cell concentration is 1-2 .times.10.sup.8 cells/ml of must/broth.
A higher yeast cell concentration can, for example, be arranged by immobilizing the yeast on a suitable carrier packed in a column. When the fermenting must is passed through the column packed with immobilized yeast, the number of yeast cells in contact with the volume of must in the reactor is greatly increased. This greater contact area results in a faster fermentation. The column reactor also reduces the ethanol inhibition since the ethanol containing product is continuously removed from the yeast bed. This reduction is greater as an ideal plug flow situation is approached.
It is known that yeast cells can be entrapped in calcium alginate and the resulting immobilized yeast can be used for fast fermentations. Numerous literature references describe this technique as it is applied usually in laboratory scale. Some efforts, however, have also been made to commercialize this technique. One of the best known is probably that of Kyowa Hakko in Japan, where alginate immobilized yeast is used for feed ethanol production.
For commercial scale operations, a major difficulty with alginate entrapment is the manner in which the particles are formed. It must be carried out at the production site where a yeast slurry and a solution of sodium algonate are mixed together. When this mixture is then fed into a calcium-salt solution, the alginate precipitates and at the same time occludes the yeast cells within the precipitated particles. The particles usually are in the form of droplets/beads.
A process plant that utilizes alginate entrapped yeast must have specially designed equipment just to produce these beads. Furthermore, there is a potential risk for contaminating the yeast with wild microorganisms. This is especially critical when production of an alcoholic beverage flavor is important (e.g., beer, wine, cider and similar kinds of products). For technical/fuel ethanol production the criteria of contamination is not as important as in the consumables although it does affect the productivity.
A second major difficulty for alginate beads used on a commercial scale is in the physical strength of the beads. The beads are soft and easily compressible. Operating large fermentation columns can be a problem and fast downflow process streams are difficult to handle. On the other hand, a typical upflow mode in fermentation greatly wears the beads. Also, to run a reactor downflow under pressure with compressible material is virtually impossible.
A third difficulty with entrapment is the diffusion limitations which slow down the accessibility of the substrate in contact with the yeast inside the bead.
Finally, if the system becomes contaminated or otherwise disturbed so that a continuous operation must be discontinued, the whole lot of column material (alginate together with the yeast) must be discarded. No reuse is possible.
Therefore, it is an object of the present invention to develop a method for continuous column fermentation that does not require on-site formation of the column material. Another object is to develop a method that eliminates sources of contamination. Yet another object is to develop a method of this type that can withstand pressure. Still another object of this invention is to develop a method where the carrier material can be regenerated and reused. Further objects include development of processes that produce alcohol at reasonably rapid rates such as at least a minimum of about 1 bed volume of about 5 percent alcohol per day.